A 5Gh/s miner could be profitable if you had one now, or maybe in the next few months or so. But by that time many other people would also have their own ASICs, driving difficulty way up. The estimate of 3 weeks is based on a static difficulty. Furthermore, there are _thousands_ of people with orders already. BFL has to clear through that backlog, and that will take time. By that time the chance of them being profitable is pretty low.